So my 03 gt I bought in January this year. It has 160k very well maintained highway miles. Ok when I take off in early morning after about 2 or 3 minutes warm up, it will absolutely throw u into the seat hard (has nitto 555s so it just grips super awesome) once it warms up a little she gets a little lazy. Once fully warm and if its 85+ degrees outside it just lacks that seat of the pants acceleration by far. It still runs fine with no backfire or surging, it just becomes lazy. Some mornings feel quicker tho but its always quicker when cold by far. A little background : when I first got it it would get sideways in 3rd but it was cold and wet so I'm not worried, it just grips excellent when hot and dry (maybe lack of power, but takeoffs in the morning would have plenty of power to spin them if they sucked) I put an intake on shortly after (had a coolant leak) but didn't notice any power loss, but at the time I wasn't driving it hard. Also I have an o/r x and flows also have no codes. New fuel filter, plugs, knn air filter, oil and filter. Fuel pressure over 40psi. Engine is all stock, stock gears, also the airbox snorkel is gone (would this be a problem in heat?) my questions: is this normal for a stock gt? Something with the intake that won't set codes? Should the rear be slidin around with the nittos, or is something wrong?I would just like to keep that power I have when cold. What do y'all think I should do?

Test your alternator with a volt meter. I know that sounds weird but mine was just gutless in the day and it felt as if it wasnt getting enough fuel, couple weeks later it had issues starting...replaced the alt and it runs strong even in the summer heat

Leakdown as see if your rings are holding compression in the cylinders. It would run better cold if there is a ring issue because the oil is thicker. Thicker oil would not get past the rings if the ring is just starting to go but it will when the oil heats up and is thinner.

I'm just speculating here. Google "leakdown check" for more details on how to do that but I would definitely check compression and the rings on a 160k motor.

You also should see if you can find out if the COPs were ever replaced. If they are original at 160k, that could cause issues too.

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2000 Mustang GT Steeda #0048

Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.